1. Field of the Invention
This invention relates generally to the field of reflective optical materials and, more particularly, to a process for forming graded index optical materials for reflective diffraction optical elements, such as optical filters and reflective or transmissive optical coatings.
2. Description of the Prior Art
In various optical systems, it is often necessary to provide a filter in order to remove undesired radiation while at the same time allowing desired radiation to be efficiently transmitted or reflected. Such filters and coatings are used, for example, to provide protection from laser radiation for personnel, for electro-optical detectors, and for optical mirrors in a laser system, as a holographic lens in a head-up display system, or in night vision visors. The optical filters currently used for such purposes include absorption filters, reflective multiple layer dielectric filters, and diffraction filters generated by optical holographic techniques. However, each of these approaches to providing optical filters has certain disadvantages, as discussed below.
The absorption filter comprises a material which is impregnated with absorption dyes or materials with intrinsic absorption at the wavelength of the incoming laser radiation, as described, for example, in the book entitled "Handbook of Optics", W. G. Driscoll, ed., McGraw-Hill Book Co., New York, 1978, in Section 8 (Coatings and Filters), at pages 7 to 32. This type of protection has the serious disadvantage that the absorbing dye decreases the amount of transmitted radiation to unacceptably low levels. In addition, for laser applications, as the laser radiation energy increases, the radiation can damage the protective filter itself.
The reflective multiple layer dielectric filters typically consist of alternate layers of two dielectric materials of different refractive indices, which are formed on the surface of a substrate by known deposition techniques, such as chemical vapor deposition, sputtering, or thermal evaporation. When the optical thickness of each layer is chosen to be one-quarter of the wavelength of the radiation being reflected, such a structure is referred to as a "quarterwave stack", as discussed, for example, in U.S. Pat. No. 4,309,075 and in the book entitled "Handbook of Optics", previously referenced, in particular in Section 8. However, there are limitations on the spectral bandwidths which can be achieved by such structures, because of the limited material combinations available and the resulting restriction on the choices of index modulations. Moreover, defects at the abrupt interfaces between the layers in a multilayer structure can cause unwanted optical scattering. In addition, these defects can cause excessive absorption of radiation by the dielectric material, which can result in thermal damage to the optical filter. Furthermore, in a multilayer dielectric coating, the electric field is strongest at the interface regions between the high index material and the low index material. This highly localized field occurring at the abrupt interfaces can produce maximum temperature increases. Since the thermal expansion coefficients are different for the different dielectric materials of adjacent layers, high thermal stress is developed at the interface regions, which could cause delamination of the successive layers in the film. In addition, the high thermal stress could create microscopic dislocations which result in unwanted optical scattering by the film. Further, substrate roughness, pinholes and contaminants in the conventional multilayer structures formed by evaporation or sputtering techniques increase absorption and scattering, generate localized heating, reduce maximum reflectivity, and increase radiation damage. Finally, these multilayer coatings exhibit reflectance peaks at multiple wavelengths, which causes reduced optical transmission.
Diffraction optical elements have been generated using known methods of optical holography in photosensitive gelatin material, as discussed, for example, in the book entitled "optical Holography", by Collier, Burckhardt, and Lin, Academic Press, New York, 1971, Chapter 9 (Diffraction from Volume Holograms) and Chapter 10 (Hologram Recording Materials), as well as in the book entitled "Handbook of Optical Holography", by Caulfield, Academic Press, New York, 1979, Chapter 10 (Application Areas). However, gelatin diffraction elements have environmental stability problems and are susceptibile to degradation by humidity and heat. In order to overcome this problem, a protective layer such as glass or a glass-like coating can be used, but such a layer complicates the manufacturing process and adds to unit cost. Moreover, such gelatin filters are limited to use for radiation in the wavelength range from the visible to the near infrared since sensitized gelatin is not sensitive to longer wavelength exposures. Consequently, filters for infrared applications cannot be fabricated in a gelatin structure. In addition, the index modulation in the gelatin, which is produced by exposure to the holographic interference pattern and subsequent development, is limited to a shape approximating a sinusoidal configuration or a roughly superimposed multiple sinusoidal configuration. Furthermore, the fabrication of a gelatin filter requires numerous steps, in particular numerous wet chemical steps for development, which are sensitive to processing variables, such as temperature or vibration, that affect the efficiency and peak wavelength of the final structure. In addition, since the resistance of gelatin to damage by heat or radiation is relatively low, gelatin filters are limited to low power applications. Finally, fabrication of a filter which reflects radiation at two selected wavelengths requires multiple exposure of the gelatin to two holographic patterns, which produces an irregular index profile that reduces the efficiency of the filter.
The present invention is directed to the fabrication of a high-efficiency diffraction optical element which alleviates the prior art problems discussed above, which are due to either the structure of the optical element or the process used for its fabrication.